1. Field of the Invention
The present invention relates to a precision quenching apparatus and method with induction heating, particularly, for producing a predetermined mottled pattern of a plurality of rows of individually quenched layer portions in an inner peripheral surface of a cylinder in a cylinder block of an engine, for example such as diesel engines and the like used under heavy loads.
2. Description of the Prior Art
For example, in an inner peripheral surface of a cylinder of a diesel engine used under a heavy load, there is formed a mottled pattern of individually quenched layer portions through a conventional inner surface quenching method. In this connection, the applicant of the present application has already proposed an induction heating coil and an induction quenching method in Japanese Patent Laid-Open Nos. Hei 7-161461 and Hei 7-272845.
However, such conventional quenching method with induction heating is disadvantageous in that: when a center of the cylinder deviates from that of the induction heating coil to fail to keep constant a clearance between an inner peripheral surface of the cylinder and an outer peripheral surface of the induction heating coil, a plurality of concyclic portions of the cylinder to be quenched differ in temperature from each other after heating, which makes it impossible to produce a constant pattern of quenched portions of the cylinder, the quenched portions differing from each other also in quenched depth.
For example, as for a quenching of the cylinder having an inner diameter of 80 mm, experiments have shown that the above-mentioned clearance must be 1.25 mm plus or minus 0.15 mm, and the deviation in center of the heating coil from the cylinder must be up to 0.03 mm. Consequently, in order to produce the above-mentioned constant pattern of quenched portions of the cylinder, it is necessary for the induction heating coil to position its center within a radius of several tens of .mu.m from the center of the cylinder when the quenching of the cylinder is carried out. However, such positioning operation is very cumbersome when an engine's cylinder block has a plurality of the cylinders to be quenched, and, therefore not suitable for a quenching method in mass production.
In general, water or aqueous solution is used as a cooling medium or cooling liquid in the quenching operation of the cylinder. When the cooling liquid is too low in temperature for the quenching operation of the cylinder, distortion and quenching crack of the cylinder tend to occur. On the other hand, when the cooling liquid is too high in temperature, the cooling liquid is poor in cooling power, and, therefore fails to sufficiently quench the cylinder.
Consequently, it is necessary to keep the cooling liquid at a temperature of from 30 to 40 degrees centigrade during the quenching operation. However, as is clear from the above, such temperature of from 30 to 40 degrees centigrade of the cooling liquid is higher than room temperatures. As a result, a quenching apparatus used in this quenching operation increases in temperature, and, therefore thermally expands because of its increasing temperature. Consequently, a base frame of the apparatus also expands to deviate from its initial set position, which makes it impossible to realize a precision quenching operation. Namely, the temperature of the entire apparatus is equal to room temperature before the start of the quenching operation.
Under such circumstances, when the cooling liquid having a temperature of from 30 to 40 degrees centigrade, which is higher than the room temperature, is discharged into and from a cooling liquid tank of the apparatus each time the quenching operation is conducted, the base frame of the apparatus increases in temperature to thermally expand, which causes a center of the cylinder to deviate from a corresponding center of the induction heating coil. As a result, the apparatus fails to keep constant the clearance between an inner peripheral surface of the cylinder being quenched and an outer peripheral surface of the induction heating coil on the same circumferential plane, which makes it impossible for the above-mentioned mottled pattern of the cylinder to be uniformly produced.
In order to overcome the above problems, it is possible for the quenching apparatus to employ a suitable control means, in which a value representing the above-mentioned deviation of the center of the cylinder is incorporated to compensate for such deviation occurring in the quenching operation. The apparatus of this type is used after it reaches a predetermined working temperature. However, such apparatus takes too much time to reach such working temperature. Particularly, in winter, it is necessary to warm up the apparatus by discharging the cooling liquid having a temperature of from 30 to 40 degrees centigrade into and from the cooling liquid tank, which takes too much time and labour, and, therefore forces the operators of this apparatus to do such warm-up job in the early morning. This is disadvantageous from the economical point of view. In order to overcome the above problem, the quenching apparatus may be disposed in a constant temperature chamber. Alternatively, a suitable chiller or chilling machine for precisely controlling the cooling liquid in temperature may be used. However, both the constant temperature chamber and the chiller are expensive in installation, and, therefore also disadvantageous from the economical point of view.
Further, in case that a plurality of rows of the mottled pattern of individually quenched portions of the cylinder are produced, the cooling liquid gradually increases in temperature during quenching operation. Consequently, the quenched layer portions of the cylinder in the first row are always smaller in quenched depth than those in the subsequent row. This is a problem inherent in the conventional quenching method and apparatus.